NOAA meets on benefits of improved 3D positioning

Nautical charts have all a horizontal datum.
On a global basis, the International Hydrographic Organization (IHO) designated the use of the World Geodetic System (WGS) as the universal datum.
Since then, the horizontal features have been based on WGS 84 or in other geodetic reference systems which are compatible, such as NAD 83 (in the US) or the ITRF combined with the GRS80 ellipsoid.

But what about heights ?

Nautical charts have depths referred to different tidal surfaces, which may vary from chart to chart. Tide heights are given relative to the "datum" which in most cases is one of different benchmarks corresponding to low tides of varying extremeness.
In the United States, Mean Lower Low Water (MLLW) is the typical low water reference surface (MLWS in Nederlands, LAT in Germany...)
By the way, to support harbor and river navigation, bridge clearances are also typically referenced to mean high water (MHW): not to a low water...

Vertical datum used by software covering the whole planet (such as Google Earth) is the WGS84 EGM96 Geoid which gives altitudes in meters above sea level (MSL) while seafloor mapping software (Hypack, Olex...) reference bathymetric survey data to Low Water vertical datums...

In a goal of uniformity, seamless VDatum software emerges to cover all of the U.S. coastal areas out to 25 nautical miles from land. The availability of VDatum nationwide will enable bathymetric, topographic and coastline data to be easily transformed and assembled in a manner that complements dissemination through national databases.

Last week the National Oceanic and Atmospheric Administration held a Federal Geospatial Summit in Maryland to discuss the proposed improvements to the National Spatial Reference System (NSRS) in the US.
The meeting discussed how the replacement of the North American Datum of 1983 (NAD83) and the North American Vertical Datum of 1988 (NAVD88) will positively impact measurement and operations.

The agency recently received the findings of an independent study that shows the benefits to the U.S. economy for better positioning.
The study found that the NSRS provides a benefit of more than $2.4 billion annually to the U.S. economy, and that an additional $522 million in economic benefits could be realized by improving the precision of elevation measurements, with an estimated $240 million saved by improved floodplain management.

The existing vertical datum means elevation errors from 16 inches to 6 feet at sea level.
The improved Gravity for the Redefinition of the American Vertical Datum (GRAV-D) reference system would improve that error to under an inch.

Links :

• overview of the 'U.S. economy for better positioning' study / full report
• NSRS / Elements of NSRS
  
• NOAA : Tutorial geodesy
• Height modernization program
  
• Water level information

Image of the week : plankton arrives in Scandinavia

Envisat's MERIS acquired this image on 3 May 2010 at a resolution of 300 m.

Envisat captures a crescent-shaped string of plankton in the North Sea weaving through the Scandinavian region. Norway (left) and Sweden (right), part of the Scandinavian Peninsula, are visible at the top, and Denmark is at bottom right.

The emerald green lake seen in Sweden is Vänern, the largest in the country..
The green water around Denmark is due to sediments being transported in the water.
Also visible (image centre) is Norway’s second largest fjord, Hardangerfjord at the north of Bergen.

The plankton, which forms the most abundant life in the oceans, is mainly composed of microscopic marine plants that drift on the surface of the sea or near it.
The plankton was nicknamed "the grass of the sea" because it is the staple food for a lot of other forms of marine life.
As the plankton contains pigments of chlorophyll for photosynthesis, these simple organisms play a role similar to terrestrial green plants in the process of photosynthesis.

Plankton is capable of transforming inorganic compounds such as water, nitrogen and carbon in complex organic materials. Because of his ability to digest these compounds, it is estimated that the plankton contributes as much as terrestrial vegetation to extract carbon dioxide from the atmosphere.

The chlorophyll used for photosynthesis by these microscopic organisms gives color to the waters of the ocean where they concentrate, which provides a means of detecting from space through sensors dedicated to the study of "ocean color" as the camera MERIS (Medium Resolution Imaging Spectrometer) Envisat.

Links :

• Envisat’s MERIS captures phytoplankton bloom (2002)
• Earth from Space: 'The grass of the sea' (2006)
• NASA : Phytoplankton bloom in the North Atlantic
  
• NASA : Phytoplankton Bloom in the Arabian Sea
  

Marine GeoGarage sightseeing contest : win a 'Chart Premium' subscription

Following to our previous post :

As Marine GeoGarage gives to the user the possibility to superimpose official raster charts from International Hydrographic Services upon Google aerial and satellite imagery, it could be interesting to report sights with major differences.

see : NOAA shoreline website

Aerial and satellite Google images are regularly updated with accuracy improvements (see the Historical imagery tool in Google Earth), so the use of the Marine GeoGarage nautical chart opacity tool could become a basic technique for coastal change monitoring.

User feedback :

Users who note a major difference between imagery and nautical chart can report us (contact@geogarage.com) the place using the network link at the bottom left side of the viewer.
http://marine.geogarage.com/routes?anonmap=...........

GeoGarage contest :

We will publish a specific page signaling the different reports (with reference to the user or in an anonymous way).
Every month, the GeoGarage team will select the most significant report and will credit a month 'Premium Chart' for the winner.

Aerial photography and shoreline mapping : checking accuracy

Example of difference between aerial imagery and chart
(Ngmararu point in New Zealand)

Aerial photographs are the primary source material used to create coastal survey maps.
These data sets, in turn, provide information to Hydrographic Services for producing official nautical charts.

Combining information from aerial photographs with hydrographic data helps to ensure that nautical charts are accurate.
Nautical charts are one of the most fundamental tools available to mariners for planning voyages and navigating ships using the shortest, safest, and most economical routes.
So it is obviously important that the information displayed on charts is correct.

Notes about accuracy :

• regarding nautical charts :

When international Hydrographic Services conduct hydrographic surveys, they used the best technology through stringent accuracy standards for the collection of data by their survey vessels. But of course only for the moment in time.
Coastal waters are extremely dynamic. Natural shoaling occurs, earthquakes move sea beds, channels are dredged and new wrecks and obstructions are discovered.

So the necessity for our web service to propose regular chart updates (which should match at term to the frequency of updates proposed by the different Hydrographic Services) and for the user to take into account the information promulgated through Local Notice to Mariners.
Don't forget that for surveys performed prior to the mid 1990’s (so before standard DGPS), the accuracy requirement was only 1.5mm at the scale of the survey. On a 1:20,000-scale harbor approach survey, an accuracy of 1.5mm on the chart equates to 30 meters in real life.

RNCs are made by scanning the paper chart printing materials.
Any inaccuracies due to old methods of collecting, processing and displaying data on the paper chart were transferred to the RNCs. As a result, the accuracy of modern positioning systems such as GPS may exceed the positional accuracy of the RNC. The impact of positioning accuracies can be minimized by not zooming an RNC beyond the scale of the original chart.
For example, while NOAA has accuracy standards for each step in the data collection and chart production process, much of the depth information found on official nautical charts is based on surveys conducted before 1940, the shoreline is more than 20 years old, and paper charts used to be compiled manually.

By the way, this also means that the prudent mariner must use the Route planning tool on the Marine GeoGarage viewer to set waypoints to pass shoals or isolated dangers with utmost caution, no matter what navigation method is used. For navigation purposes, it's always necessary to keep a safety margin...

• regarding Google Maps imagery :

The vertical aerial photographs used in Google Maps have been georeferenced to align with the coordinate system.
The process of georeferencing involves identifying ground control points in the image for which accurate coordinates are available. A transformation is then calculated by computer software which processes the image so that it aligns to the ground coordinate system (Wolf and Dewitt, 2000).
Mosaics are used to stitch many aerial photographs together.

Controlled mosaics use rectified photos so that all of the photos are vertical and at the same scale. In mosaic assembly, image positions of common features in adjacent photos are matched as closely as possible. A plot of control points is used to match and constrain positions, similar to the technique used in georeferencing.
Uncontrolled mosaics simply match the image details of adjacent photos without using the ground control, which is quicker but less accurate in terms of the coordinate reference system.
Semicontrolled mosaics have either no ground control or use photos that have not been rectified.

So Google Maps are certainly not to be considered a "gold standard" for accuracy as they provide a seamless worldwide imagery display. Google buys or licenses the imagery from different sources using different quality standards and referred to different datums, to stitch it all together to make up their product in an unique WGS84 datum.

Links :

• Validation of Internet satellite images mosaics and e-charts
• The Coast and Shoreline Change Analysis Program (CSCAP)
  
• NOAA : Differential GPS (DGPS) & your chart
• AHS (Mike Prince) : Accuracy and reliability of charts
• NGA : Using nautical charts with GPS
  

Seamounts identified as significant, unexplored territory

Fly-though showing the seamounts of the Phoenix Islands Protected Area (PIPA) in the Central Pacific
(video courtesy Greg Brost and Peter Etnoyer, Aquanautix Consulting)

From ScienceDaily

Scientists from NOAA and Texas A&M University-Corpus Christi were astounded to find that seamounts, mountains that rise from the seafloor, rank as some of the most common ocean habitats in the world.

Their findings are published in a new study and reverse previous beliefs about the prevalence of seamounts, which are treasure troves of marine biodiversity.
"Unlike beaches or even coral reefs, most people will never see a seamount, but this study shows that they are clearly one of the predominant ecosystems on the planet," said Peter Etnoyer, Ph.D., principal investigator of the study and marine biologist at NOAA's Center for Coastal Environmental Health and Biomolecular Research.
"We can only hope that through this study, people begin to realize what a vast unknown the ocean represents, and what a vital role it plays on Earth."

Although researchers have thoroughly explored some 200 seamounts and mapped and sampled a hundred others, this study is the first to estimate that more than 45,000 seamounts dot the ocean floor worldwide -- a total of roughly 28.8 million square kilometers or an area larger than the continent of South America.
The discovery was made possible using satellite altimetry data that measured incredibly slight changes in the sea surface height that, along with statistical analysis models, indicated the presence of these submerged mountains.
"Seamounts are biodiversity 'hotspots', with higher abundance and variety of life forms than the surrounding seafloor," said Tom Shirley, Ph.D., contributing author of the study and a conservation scientist with the Harte Research Institute at Texas A&M University-Corpus Christi. "
In fact, new species are observed or collected on nearly every submersible dive."
Two dozen new species of corals and sponges, for example, have been collected from seamounts in the Gulf of Alaska since 2002.

Seamounts not only make up the largest area of ocean habitat, they are also highly productive environments that can serve as habitats for important commercial fish species like orange roughy and sablefish.
This research, which is the first-ever comparison of the size of oceanic and land habitats, is featured in the journal Oceanography.

Links :

• Encyclopedia of Earth
• Seamount Biogeosciences Network (SBN)
  
• Seamounts online
  
• ICES CIEM